Elective myocardial revascularization has been shown to reduce post-operative infarction and improve survival in patients with coronary artery disease, but, emergency coronary bypass surgery for the treatment of acute or impending infarction has produced only limited clinical success. The short time within which revascularization must be completed in order to salvage ischemia may account for poor results. The demonstrated effectiveness of hypothermia as a method of protecting a globally ischemic heart during cardiac surgery suggests that hypothermia could also be valuable as a preoperative treatment for patients with acute or impending infarction. Hypothermia favorably affects post-infarction hemodynamics, and our own studies show that whole-body cooling may inhibit development of ischemic injury following acute coronary occulusion. Preoperative hypothermia might increase the time during which effective myocardial revascularization could be accomplished and stabilize the patient's condition prior to surgery. However, hypoxic vasodilatation might be impaired by hypothermia. This could result in decreased oxygen delivery to an acutely ischemic region and increased myocardial damage. Thus, hypothermia may be detrimental rather than useful before or during aortocoronary bypass surgery. The important question of hypothermia's effect on myocardial blood flow during regional ischemia remains unanswered. This study will measure the effect of whole-body moderate hypothermia on blood flow to an ischemic region after acute coronary occlusion in dogs. Regional blood flow in different transmural layers will be measured as a function of the duration of ischemia and the myocardial temperature. The extent of myocardial injury will be evaluated by macroscopic dehydrogenase enzyme-mapping and related to the changes observed in blood flow, and hemodynamic responses to ischemia and cooling will be documented. The information from this study will clarify the value of hypothermia as a method of protecting acutely ischemic myocardium before and during emergency aortocoronary bypass surgery.